To add to Gauss's comment: to have the character sheet make a roll and modify something on the sheet at the same time requires a feature called Custom Roll Parsing. This was introduced after most sheets were created, so they don't include it. Before CRP, sheets could make a roll or modify the sheet but not both at the same time. CRP is a complex feature, so many sheet authors don't include it because it's a lot of work to add it, and they might not know how to. So while it's possible to do, it's not available for every sheet. If you want it for the CoC7 sheet, the best bet is to find that sheet's designers and ask if theiy'll include it.
